1. Field of the Invention
The present invention relates to an image forming apparatus such as a copying machine, a laser beam printer and the like, for transferring an image formed on an image bearing means onto a transfer material.
2. Related Background Art
FIGS. 14 and 15 schematically show a conventional color image forming apparatus using an intermediate transfer belt as an intermediate transfer member.
The color image forming apparatus shown in FIGS. 14 and 15 comprises a photosensitive drum (image bearing member) 1, an intermediate transfer unit 5, and a secondary transfer unit 50. The intermediate transfer unit 5 and the unitized photosensitive drum 1 can be mounted to and dismounted from a body of the image forming apparatus along a direction shown by the arrow R1. Further, the intermediate transfer unit 5 can be engaged by and disengaged from the photosensitive drum 1.
The intermediate transfer unit 5 includes triangular intermediate transfer frames 5A disposed in the vicinity of left and right end portions (in the vicinity of both axial ends) of the photosensitive drum 1, a drive roller 5b rotatably supported by the intermediate transfer frames 5A, a secondary transfer counter roller 5c, a driven roller 5d, an endless intermediate transfer belt 5a mounted on and wound around these rollers, and a first transfer roller 5j for urging the intermediate transfer belt 5a against the photosensitive drum 1 from a rear side to form a first transfer nip N.sub.1 therebetween.
The secondary transfer unit 50 includes a secondary transfer frame 21 pivotally supported by a rock shaft 22 for rocking movement in a direction shown by the arrow R50, bearings 20 supported at both left and right ends of the secondary transfer frame 21 for vertical movement, a transfer roller 11 having a central shaft 11a rotatably supported by the left and right bearings 20, secondary transfer roller pressurizing springs 19 for biasing the respective bearings 20 upwardly, and an eccentric cam (engaging/disengaging means) 18 rotated in a direction shown by the arrow R18 to lift the secondary transfer frame 21 in the direction R50, thereby urging the secondary transfer roller 11 against the intermediate transfer belt 5a to form a secondary transfer nip N.sub.2 (FIG. 15) therebetween.
In the color image forming apparatus having the above-mentioned construction, in a condition that the secondary transfer unit 50 is retarded to a retard position shown in FIG. 14 and the secondary transfer roller 11 is spaced apart from the intermediate transfer belt 5a, the photosensitive drum 1 is rotated in a direction shown by the arrow R1 and the intermediate transfer belt 5a is rotated in a direction shown by the arrow R5, so that yellow color, magenta color, cyan color and black color toner images successively formed on the photosensitive drum 1 are first-transferred onto the intermediate transfer belt 5a successively in a superimposed fashion by applying first transfer bias to the first transfer roller 5j at the first transfer nip N.sub.1.
Then, the eccentric cam 18 is rotated in the direction R18 to position the secondary transfer frame 21 to a transfer position shown in FIG. 15, and the secondary transfer roller 11 is urged against the intermediate transfer belt 5a to form the secondary transfer nip N.sub.2. In synchronous with the intermediate transfer belt 5a, a transfer material conveyed by a pair of regist rollers 7d is supplied to the secondary transfer nip N.sub.2, where the four color toner images on the intermediate transfer belt 5a are secondary-transferred onto the transfer material collectively by applying secondary transfer bias to the secondary transfer roller 11.
After the secondary transferring, the transfer material is sent to a fixing device (not shown), where the toner images are fixed to the transfer material with heat and pressure. In this way, the image formation is completed.
In the above-mentioned conventional technique, the positioning of the secondary transfer roller 11 with respect to the intermediate transfer belt 5a is achieved by rotating the eccentric cam 18 to urge the secondary transfer roller 11 against the intermediate transfer belt.
Next, an example of an image forming apparatus using an intermediate transfer drum as an intermediate transfer member will be explained with reference to FIGS. 16 and 17.
Four color toner images successively formed on a photosensitive drum 1 rotated in a direction shown by the arrow R1 are successively first-transferred onto an intermediate transfer drum 5B (rotated in a direction shown by the arrow R5) in a superimposed fashion. Then, the four color toner images are collectively secondary-transferred onto a transfer material supplied from a pair of regist rollers 7d by means of a secondary transfer unit 50. The secondary transfer unit 50 includes a rockable secondary transfer frame 24A, rollers 11A, 11B, 11D rotatably supported by the secondary transfer frame 24A, a secondary transfer belt 11C mounted and wound around these rollers, a secondary transfer frame pressurizing spring 25A for biasing the secondary transfer frame 24A toward the intermediate transfer drum 5B, and an eccentric cam 18A rotated to lift or lower the secondary transfer frame 24A. The secondary transfer frame 24A can be rocked around a shaft of the roller 11B. Upon the secondary-transferring of the toner images onto the transfer material, when the eccentric cam 18A is rotated by about a half revolution from a position shown in FIG. 16 to a position shown in FIG. 17, a tip end (right end in FIG. 16) of the secondary transfer frame 24A is lifted, with the result that the roller 11A is urged against the intermediate transfer drum 5B to form a secondary transfer nip N.sub.2 between the intermediate transfer drum SB and the secondary transfer belt 1C. While the transfer material is being passed through the secondary transfer nip N.sub.2, the toner images on the intermediate transfer drum 5B are transferred onto the transfer material collectively by applying secondary transfer bias to the roller 11A.
The above-mentioned conventional technique (FIGS. 14 and 15) in which the intermediate transfer belt 5a is used as the intermediate transfer member, the positioning of the secondary transfer roller 11 with respect to the intermediate transfer belt 5a is achieved by rotating the eccentric cam 18 to urge the secondary transfer roller 11 against the intermediate transfer belt.
Thus, the positional accuracy of the secondary transfer roller 11 with respect to the intermediate transfer unit 5 depends upon attachment accuracy from a rock shaft 22 of the secondary transfer frame 21 to the secondary transfer roller 11 and dimensional accuracy of the eccentric cam 18, with the result that it is difficult to keep the correct positional accuracy regarding the up-and-down direction and the left-and-right direction in FIGS. 14 and 15. Further, regarding the secondary transfer roller 11, it is difficult to maintain the parallelism of the intermediate transfer unit 5 with respect to the secondary transfer counter roller 5c. It is particularly noticeable in an apparatus in which the intermediate transfer unit 5 and the photosensitive drum 1 can detachably mounted on the apparatus body and an apparatus in which the intermediate transfer member can be engaged by and disengaged from the photosensitive drum 1.
Thus, the urging force of the secondary transfer roller 11 against the intermediate transfer belt 5a becomes unstable to generate unevenness in the urging forces at the left and right ends of the secondary transfer roller 11, poor image due to fluctuation of pressure during the secondary-transferring and/or skew-feed of the transfer material. Further, the penetration of the transfer material into the secondary transfer nip N.sub.2 becomes unstable to generate incorrect positioning of an image tip and/or sheet jam.
Incidentally, such problems occur in the conventional technique (FIGS. 16 and 17) in which the intermediate transfer drum 5B is used as the intermediate transfer member. Further, such problems occur similarly in mono-color image forming apparatuses in which a toner image on a photosensitive drum is transferred onto a transfer material at a transfer nip between the photosensitive drum and a transfer roller.